1. Field of the Invention
The present invention relates to a method for polishing a surface (surface to be polished) of an object, such as a semiconductor wafer, into a flat mirror surface, and more particularly to a method and an apparatus for polishing an object, which are useful for polishing and removing an extra metal interconnect material other than that embedded in trenches in a damascene interconnect formation process for a semiconductor device.
2. Description of the Related Art
For the formation of interconnects of a semiconductor device, a so-called damascene process is currently used which involves filling, by plating, a metal interconnect material (conductive material) such as aluminum, or more recently copper or silver, into trenches or contact holes previously formed in an insulating film (interlevel dielectric film), and then removing an excess of the metal interconnect material by chemical mechanical polishing (hereinafter abbreviated as “CMP”).
FIG. 1 illustrates an exemplary damascene process for forming copper interconnects. First, fine trenches 202a having a small width and wide trenches 202b having a wide width are formed in an insulating film (interlevel dielectric film) 200 of, e.g., SiO2 or a low-k material, deposited on a surface of a substrate W such as a semiconductor substrate, and then a barrier metal layer 204 of, e.g., TaN is formed on an entire surface of the substrate. Thereafter, a seed layer (not shown), which serves as a feeding layer during electroplating, is formed on a surface of the barrier metal layer 204, as necessary. Next, copper plating of the substrate surface is carried to form a copper film 206 on the surface of the substrate W, thereby filling the copper film 206 into the trenches 202a, 202b. Thereafter, an extra copper film 206 and barrier metal layer 204 on the insulating film 200 are removed by chemical mechanical polishing (CMP) into a flattened surface, thereby forming fine interconnects 208a and wide interconnects 208b, composed of copper, in the insulating film 200, as shown in FIG. 8D.
In the formation of the copper film 206 by copper plating on the surface of the substrate W where the fine trenches 202a and the wide trenches 202b are co-present, plating tends to be promoted whereby the copper film 206 becomes raised over a fine trench 202a, whereas promoted growth of copper does not occur in a wide trench 202b whereby the copper film 206 becomes recessed over the wide trench 202b. As a result, as shown in FIG. 1, a surface level difference (irregularities) H1, which is the sum of the height of the raised portion (mounding) over the fine trench 202a and the depth of the recessed portion (dishing) over the wide trench 202b, is produced in the copper film 206 formed on the substrate W.
Though the surface level difference (irregularities) H1 in the copper film 206 after plating gradually decreases as polishing of the copper film 206 by CMP progresses, a level difference H2 remains in the recessed portion (dishing), corresponding to the wide trench 202b, of the surface of the copper film 206, as shown in FIG. 2. It is generally difficult to eliminate the level difference H2. Accordingly, when removing the extra copper film 206 and barrier metal layer 204 on the insulating film 200 to form the fine interconnects 208a and the wide interconnects 208b, dishing (over-polishing) will occur in the surfaces of the wide interconnects 208b. 
Such dishing is influenced by the elasticity of a polishing pad and the polishing pressure applied during CMP. A polishing pad, whose surface is roughened by a diamond-electrodeposited dresser, is generally used in CMP in order to maintain a constant polishing rate. A polishing liquid (slurry) containing an abrasive is allowed to intrude into recesses of the roughened surface of such a dressed polishing pad upon CMP. A film of metal interconnect material such as the copper film 206, deposited in excess, can be polished away by pressing the polishing pad, with the polishing liquid held on the surface, against the film of metal interconnect material formed on an object, such as a substrate. However, the polishing pad having the roughened surface can easily enter the recessed portions of the film of metal interconnect material, such as the copper film 206, having a surface level difference (irregularities), whereby not only the surfaces of raised portions but also the bottoms of recessed portions can be polished. Accordingly, though the surface level difference may be reduced, it will not be eliminated.
In order to reduce a surface level difference (irregularities) in an object film, such as a film of metal interconnect material, as much as possible by CMP, it is conceivable to polish only the surfaces of raised portions of the object film without polishing the bottoms of recessed portions. To this end, it may be considered to use a most rigid polishing pad so that the polishing pad will make contact with only raised portions of the object film and not with the bottoms of recessed portions. In this regard, it is known to use a rigid single-layer polishing pad instead of a two-layer polishing pad (upper layer: rigid polishing pad, lower layer: an elastic material such as polyurethane foam) commonly used in CMP.
It is also known that a surface level difference in an object film, such as a film of metal interconnect material, can be reduced by not using a polishing pad but using a so-called fixed abrasive, comprising abrasive grains of, e.g., cerium oxide (CeO2) fixed in a binder such as a phenol resin, in carrying out polishing of the film (see Japanese Patent Laid-Open Publication No. 2000-315665).